Molecular Magnets Based on Homometallic Hexanuclear Lanthanide

Article pubs.acs.org/IC

Molecular Magnets Based on Homometallic Hexanuclear Lanthanide(III) Complexes Sourav Das,† Sakiat Hossain,† Atanu Dey,† Sourav Biswas,† Jean-Pascal Sutter,*,‡,§ and Vadapalli Chandrasekhar*,†,# †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, F-31077 Toulouse, France § Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France # National Institute of Science Education and Research, Institute of Physics Campus, Sachivalaya Marg, PO: Sainik School, Bhubaneswar - 751 005, India ‡

S Supporting Information *

ABSTRACT: The reaction of lanthanide(III) chloride salts (Gd(III), Dy(III), Tb(III), and Ho(III)) with the hetero donor chelating ligand N′-(2-hydroxy-3-methoxybenzylidene)6-(hydroxymethyl)picolinohydrazide (LH3) in the presence of triethylamine afforded the hexanuclear Ln(III) complexes [{Ln 6 (L) 2 (LH) 2 }(μ 3 -OH) 4 ][MeOH] p [H 2 O] q [Cl] 4 ·xH 2 O· yCH3OH (1, Ln = Gd(III), p = 4, q = 4, x = 8, y = 2; 2, Ln = Dy(III), p = 2, q = 6, x = 8, y = 4; 3, Ln = Tb(III), p = 2, q = 6, x = 10, y = 4; 4, Ln = Ho(III), p = 2, q = 6, x = 10, y = 2). Xray diffraction studies revealed that these compounds possess a hexanuclear [Ln6(OH)4]14+ core consisting of four fused [Ln3(OH)]8+ subunits. Both static (dc) and dynamic (ac) magnetic properties of 1−4 have been studied. Single-molecule magnetic behavior has been observed in compound 2 with an effective energy barrier and relaxation time pre-exponential parameters of Δ/kB = 46.2 K and τ0 = 2.85 × 10−7 s, respectively.

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((8-hydroxyquinolin-2-yl)methylene)picolinohydrazide), 11 which possesses a rhombus-type geometry and shows a double relaxation in its dynamic magnetization studies. Exploiting the concept of keto−enol tautomerism and conformational flexibility of ligands, a concept introduced by Tang and coworkers,19b we prepared the series of tetranuclear derivatives [Ln4(LH)2(LH2)2(μ2-η1η1-Piv)2(η1-Piv)4]·2CHCl312 (LnIII = Dy, Tb, Gd; LH3 = N′-(2-hydroxy-3-methoxybenzylidene)-6(hydroxymethyl)picolinohydrazide), in which the DyIII derivative was shown to possess a SMM behavior with a two-step relaxation. In a continuation of these studies, we now reveal the assembly of hexanuclear lanthanide {Ln6} complexes (Ln(III) = Gd (1), Dy (2), Tb (3), Ho (4)), prepared by using LH3. Among these compounds, 2 is shown to possess slow relaxation of magnetization below 14 K. These results are discussed herein.

INTRODUCTION For the past two decades, the chemistry of lanthanide complexes has experienced a renaissance of sorts in view of the new applications of these complexes in catalysis,1 luminescent materials,2 and magnetic materials.3 In the field of molecular magnetism in general and single-molecule magnets (SMMs) in particular, heterometallic 3d/4f4 and homometallic 4f3,5 complexes have made a substantial impact, owing to the large ground-state spin of lanthanide ions and also to their substantial magnetic anisotropy.6 In fact, some of the largest energy barriers known for the reversal of magnetization among single-molecule magnets are displayed by polynuclear lanthanide complexes, as exemplified by {Dy5}7 and {K2Dy4}8 complexes. The magnetic anisotropy of lanthanide ions have also rendered them as suitable candidates in single-ion magnets (SIMs).9 We have utilized the phosphorus-supported hydrazone ligand SP[N(Me)NCH-C6H3-2-OH-3-OMe]3 and have prepared several heterometallic 3d/4f10 complexes, many of which have been shown to be SMMs; notably we also were able to demonstrate the first example of CoII2LnIII10a,b SMMs. Spurred by this, we have been examining other systems including polynuclear lanthanide complexes. Thus, we have recently reported a tetranuclear compound, [{(L′H)2Dy4}(μ2O)4](H2O)8·2CH3OH·8H2O (L′H3 = 6-(hydroxymethyl)-N′© 2014 American Chemical Society

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EXPERIMENTAL SECTION

Reagents and General Procedures. Solvents and other general reagents used in this work were purified according to standard procedures.13 Pyridine-2,6-dicarboxylic acid, sodium borohydride, DyCl3·6H2O, TbCl3·6H2O, GdCl3·6H2O, and HoCl3·6H2O were Received: January 9, 2014 Published: April 25, 2014 5020

dx.doi.org/10.1021/ic500052v | Inorg. Chem. 2014, 53, 5020−5028

Inorganic Chemistry

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Table 1. Crystal Data and Structure Refinement Parameters of 1−4 formula formula wt cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρc (g cm−3) μ (mm−1) F(000) cryst size (mm3) θ range (deg) limiting indices no. of rflns collected no. of indep rflns completeness to θ (%) refinement method no. of data/ restraints/params goodness of fit on F2 final R indices (I > 2σ(I)) R indices (all data) largest diff peak, hole (e Å−3)

1

2

3

4

C66H102Cl4Gd6N12O38 2756.90 triclinic P1̅ 12.065(5) 12.880(5) 16.967(5) 69.631(5) 89.371(5) 76.267(5) 2393.7(15) 1 1.913 4.295 1338 0.049 × 0.036 × 0.022 4.08−25.03 −12 ≤ h ≤ +14, −15 ≤ k≤ +13, −24 ≤ l ≤ +24 12467 8282 (R(int) = 0.0254) 97.8

C66H98Cl4Dy6N12O40 2816.36 triclinic P1̅ 12.042(5) 12.905(5) 16.891(5) 68.012(5) 88.667(5) 76.685(5) 2362.8(15) 1 1.979 4.887 1362 0.057 × 0.039 × 0.028 4.09−25.03 −14 ≤ h ≤ +14, −15 ≤ k ≤ +11, −20 ≤ l ≤ +19 12431 8205 (R(int) = 0.0510) 98.1

C66 H96Cl4Tb6N12O42 2824.87 triclinic P1̅ 12.063(5) 12.876(5) 16.830(5) 67.649(5) 87.699(5) 76.692(5) 2349.5(15) 1 1.997 4.662 1370 0.056 × 0.035 × 0.027 4.08−25.02 −14 ≤ h ≤ +14, −15 ≤ k ≤ +15, −20 ≤ l ≤ +11 12108 8098 (R(int) = 0.0246) 97.5

C64 H100Cl4Ho6N12O40 2808.94 triclinic P1̅ 12.059(5) 12.920(5) 16.840(5) 68.218(5) 88.492(5) 76.446(5) 2363.2(15) 1 1.974 5.164 1358 0.053 × 0.042 × 0.025 4.11−25.03 −10 ≤ h ≤ +14, −15 ≤ k ≤ +15, −17 ≤ l ≤ +20 26983 8108 (R(int) = 0.0413) 97.0

8282/38/588

8205/19/605

1.090

1.047

1.034

1.101

R1 = 0.0542, wR2 = 0.1472

R1 = 0.0725, wR2 = 0.1691

R1 = 0.053, wR2 = 0.1471

R1 = 0.0471, wR2 = 0.1248

R1 = 0.0710, wR2 = 0.1685 2.840, −1.781

R1 = 0.1127, wR2 = 0.2114 2.487, −4.034

R1 = 0.0655, wR2 = 0.1653 3.306, −2.344

R1 = 0.0643, wR2 = 0.1411 2.514, −2.064

full-matrix-block least squares on F2 8098/28/601

obtained from Sigma-Aldrich and were used as received. o-Vanillin, hydrazine hydrate (80%), and sodium sulfate (anhydrous) were obtained from SD Fine Chemicals, Mumbai, India. Methyl-6(hydroxymethyl)picolinate,14,12 6-(hydroxymethyl)picolinohydrazide,12 and o-vanillin 6-(hydroxymethyl)picolinoyl hydrazone (LH3)12 were prepared according to literature procedures. General Synthetic Procedure for the Preparation of Complexes 1−4. All of the metal complexes (1−4) were synthesized according to the following procedure. LH3 (0.06 g, 0.20 mmol) was dissolved in methanol (10 mL) and chloroform (5 mL). To this was added 1.5 equiv of LnCl3·6H2O (0.30 mmol). The reaction mixture was stirred at room temperature for 10 min. At this stage excess triethylamine (0.10 mL, 1 mmol) was added dropwise and the mixture was stirred for a further period of 10 h at room temperature to afford a clear yellow solution, which was filtered and kept for crystallization. After about 10 days, block-shaped, colorless crystals suitable for X-ray crystallography were obtained by slow evaporation from the solvent mixture. Specific details of each reaction and the characterization data of the products obtained are given below. [{Gd6(L)2(LH)2}(μ3-OH)4][MeOH]4[H2O]4[Cl]4·8H2O·2CH3OH (1). LH3 (0.060 g, 0.20 mmol), GdCl3·6H2O (0.111 g, 0.30 mmol), and Et3N (0.1 mL, 1 mmol). Yield: 42 mg, 30% (based on Gd). Mp: 180 °C dec. IR (KBr, cm−1): 3405 (b), 1613 (s), 1564 (s), 1544 (m), 1474 (s), 1443 (w), 1397 (m), 1366 (s), 1354 (m), 1239 (s), 1171 (m), 1077 (m), 1036 (s), 951 (m), 747 (s). Anal. Calcd for C66H102Cl4Gd6N12O38 (2756.90): C, 28.27; H, 3.73; N, 6.10. Found: C, 27.92; H, 3.41; N, 6.29. [{Dy6(L)2(LH)2}(μ3-OH)4][MeOH]2[H2O]6[Cl]4·8H2O·4CH3OH (2). LH3 (0.060 g, 0.20 mmol), DyCl3·6H2O (0.112 g, 0.30 mmol), and Et3N (0.1 mL, 1 mmol). Yield: 55 mg, 39% (based on Dy). Mp: 178 °C dec. IR (KBr, cm−1): 3394 (b), 1614 (s), 1570 (s), 1561 (w), 1462

8108/12/581

(s), 1452 (s), 1371 (m), 1355 (s), 1309 (w), 1222 (s), 1177 (m), 1034 (s), 950 (m), 747(s). Anal. Calcd for C 66 H 106 Cl 4 Dy 6 N 12 O 40 (2824.42): C, 28.07; H, 3.78; N, 5.95. Found: C, 27.81; H, 3.49; N, 6.23. [{Tb6(L)2(LH)2}(μ3-OH)4][MeOH]2[H2O]6[Cl]4·10H2O·4CH3OH (3). LH3 (0.060 g, 0.20 mmol), TbCl3·6H2O (0.111 g, 0.30 mmol) and Et3N (0.1 mL, 1 mmol). Yield: 58 mg, 40% (based on Tb). Mp: 175 °C dec. IR (KBr, cm−1): 3410 (w), 1612 (s), 1572 (s), 1558 (w), 1462 (s), 1453 (s), 1377 (m), 1351 (s), 1308 (w), 1239 (s), 1176 (m), 1035 (s), 951 (m), 747 (s). Anal. Calcd for C66H108Cl4Tb6N12O42 (2836.98): C, 27.94; H, 3.84; N, 5.92. Found: C, 27.85; H, 3.56; N, 6.25. [{Ho6(L)2(LH)2}(μ3-OH)4][MeOH]2[H2O]6[Cl]4·10H2O·2CH3OH (4). LH3 (0.060 g, 0.20 mmol), HoCl3·6H2O (0.113 g, 0.30 mmol) and Et3N (0.1 mL, 1 mmol). Yield: 52 mg, 37% (based on Ho). Mp: 178 °C dec. IR (KBr, cm−1): 3415 (w), 1613 (s), 1564 (s), 1544 (w), 1462 (s), 1453 (s), 1397 (m), 1354 (s), 1308 (w), 1239 (s), 1171 (m), 1036 (s), 950 (m), 747 (s). Anal. Calcd for C64H100Cl4Ho6N12O40 (2808.94): C, 27.37; H, 3.59; N, 5.98. Found: C, 27.19; H, 3.43; N, 6.21. Instrumentation. Melting points were measured using a JSGW melting point apparatus and are uncorrected. IR spectra were recorded as KBr pellets on a Bruker Vector 22 FT IR spectrophotometer operating at 400−4000 cm−1. Elemental analyses of the compounds were obtained from a Thermoquest CE instrument (CHNS-O, Model EA/110). Magnetic Measurements. Magnetic measurements were carried out with a Quantum Design MPMS 5S SQUID magnetometer in the temperature domain 2−300 K. The measurements were performed on crushed crystals from freshly isolated samples to avoid solvent loss. The powders were mixed with grease and put in gelatin capsules. The 5021


Inorganic Chemistry

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Scheme 1. (a) Synthesis of the Homometallic Ln4 Complexes by Using LH3, Where both the Conformational Flexibility and Keto−Enol Tautomerism Were Utilized,12 and (b) Synthesis of the Homometallic Ln6 Complexes 1−4, Where Exclusively the Enol Form of LH3 Is Present (This Work)

Figure 1. Tetracationic complex of 2. All hydrogen atoms have been removed, except those present in bridging hydroxide ligands or in coordinated solvent molecules. Chloride ions and solvent molecules of crystallization are not shown. magnetic susceptibilities were measured in an applied field of 1000 Oe. The molar susceptibility (χM) was corrected for the sample holder and 5022


Inorganic Chemistry

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Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for Compound 2 Bond Lengths Around Dysprosium(1) Dy(1)−O(2) Dy(1)−O(3) Dy(1)−O(9)* Dy(1)−O(12)

2.345(10) 2.384(11) 2.399(11) 2.422(9)

Dy(2)−O(9) Dy(2)−O(1) Dy(2)−O(12)*

2.324(9) 2.324(11) 2.398(10)

Dy(3)−O(7) Dy(3)−O(1) Dy(3)−O(6) Dy(3)−O(3)*

2.207(11) 2.266(10) 2.310(11) 2.355(11)

Dy(1)−O(5)*a Dy(1)−O(13) Dy(1)−N(3) Dy(1)−N(4)

2.440(13) 2.475(10) 2.493(14) 2.557(15)

Dy(2)−O(2) Dy(2)−O(12) Dy(2)−N(1)

2.403(10) 2.443(10) 2.532(12)

Dy(3)−O(9) Dy(3)−N(6) Dy(3)−O(4)*

2.453(11) 2.529(12) 2.596(10)

Around Dysprosium(2)

Around Dysprosium(3)

Bond Angles Dy(3)−O(1)−Dy(2) Dy(1)−O(2)−Dy(2) Dy(3)*−O(3)−Dy(1) Dy(3)−O(6)−Dy(1)* Dy(2)−O(9)−Dy(1)* a

110.1(4) 114.0(4) 100.3(4) 103.4(4) 107.4(4)

Dy(2)−O(9)−Dy(3) Dy(1)*−O(9)−Dy(3) Dy(2)*−O(12)−Dy(1) Dy(2)*−O(12)−Dy(2) Dy(1)−O(12)−Dy(2)

103.9(4) 97.2(3) 104.3(4) 109.7(4) 109.9(4)

Symmetry transformations used to generate equivalent atoms: (*) 1 − x + 1, −y + 1, −z + 1.

for the diamagnetic contribution of all atoms by using Pascal’s tables. ac susceptibility was measured with an oscillating ac field of 3 Oe with frequency between 1 and 1500 Hz. X-ray Crystallography. The crystal data for the compounds have been collected on a Bruker SMART CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å). The program SMART15a was used for collecting frames of data, indexing reflections, and determining lattice parameters, SAINT15a for integration of the intensity of reflections and scaling, SADABS15b for absorption correction, and SHELXTL15c,d for space group and structure determination and least-squares refinements on F2. All structures were solved by direct methods using the program SHELXS-9715e and refined by full-matrix least-squares methods against F2 with SHELXL-97.15e Hydrogen atoms were fixed at calculated positions, and their positions were refined by a riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters. The crystallographic figures have been generated using Diamond 3.1e software.15f The crystal data and cell parameters for compounds 1−4 are summarized in Table 1. The high residual density peaks in the range of 2.48−3.30 e Å−3 of all crystals are due to anomalous scattering of lanthanide ions, and the peaks are located very close to lanthanide centers (∼1 Å). CCDC-977413 (1), CCDC-977414 (2), CCDC-977415 (3), and CCDC-977416 (4) contain crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Figure 2. Hexanuclear core consisting of four fused triangular trimeric [Dy3(OH)]8+ subunits. Symmetry transformations used to generate equivalent atoms: (*) 1 − x + 1, −y + 1, −z + 1.

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RESULTS AND DISCUSSION Synthetic Aspects. Multidentate Schiff base ligands,11,16 particularly those derived from the condensation of o-vanillin Scheme 2. Binding Modes of the Ligands (L3− and LH2−) in 2

Figure 3. Dy6 core present in 2, highlighting (pink lines) an incomplete double-cubane Dy4O6 motif formed by face sharing. Symmetry transformations used to generate equivalent atoms: (*) 1 − x + 1, −y + 1, −z + 1.

and hydrazine units,17,12 have been proven to be extremely effective for the preparation of homonuclear lanthanide complexes. We have recently prepared the ligand N′-(25023


Inorganic Chemistry

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Figure 4. Different geometries around Dy(III) ions.

together four different DyIII ions (Dy3, Dy2, Dy1, and Dy3*) in a μ4-η2:η1:η2:η1:η2:η1 mode (Scheme 2). In this binding action the enolate form of the ligand is exclusively present. The hexanuclear ensemble thus formed is strengthened by the coordination action of four μ3-OH groups (Figure 1). Finally, two [LH]2− ligands, still possessing intact −CH2OH groups, function in a hexadentate manner (μ2-η1:η1:η2:η1:η1) holding two terminal lanthanide ions each (Dy3 and Dy1*) (Scheme 2). Thus, in the formation of 1−4, the ligand functions exclusively in the enolate form, unlike in [Ln4(LH)2(LH2)2(μ2η1η1-Piv)2(η1-Piv)4]·2CHCl312 (Ln = Dy, Tb, Gd), where both the keto and enolate forms of the ligand are present (Scheme 1a). Further analysis of the structure of 2 reveals some interesting features. Thus, 2 contains a hexanuclear [Dy6(OH)4]14+ core consisting of triangular trimeric motifs [Dy3(OH)]8+ that are fused with each other (Figure 2); the μ3-OH groups deviate from the lanthanide mean plane (Figure 2). The Dy- - -Dy distances found in the hexanuclear core are shown in Figure 2. The average Dy−O distance is 2.391 Å. Considering the involvement of the enolate (μ2; O2 and O2*) and μ3-hydroxide ligands, the hexanuclear core shown in Figure 3 expands into a structure containing two face-sharing incomplete double cubes (Figure 3). Finally the structure is completed by the involvement of the other coordinating groups. An inspection of the metric parameters reveals that the average bond distances Dy−μ3-Ohydroxo = 2.426 Å, Dy−μ2OPhenolate = 2.359 Å, Dy−μ2-Oenolate = 2.374 Å, and Dy−μ2Oalkoxy = 2.294 Å are similar to those found in the literature.11,12,16g,19c,d Interestingly, in the formation of 2, the two [LH]2− ligands lie approximately orthogonal to the Dy6 mean plane. In contrast, the two [L]3− ligands are nearly in the same plane (Figure S4, Supporting Information). The overall structure of 2 contains three structurally distinct DyIII ions; two of these, Dy2 and Dy3, are eight-coordinate (7O, 1N), while the other, Dy1, is nine-coordinate (7O, 2N). The shapes of these coordination spheres have been evaluated by continuous shape measures analysis carried out with SHAPE.18 The results (Figure 4 and Table S4 (Supporting Information)) reveal that Ln ions sit in environments that significantly deviate from an ideal geometry and cannot be described by a simple deformation of one of the possible geometries. The actual polyhedral shape for the eightcoordinated centers appears to be between a triangular dodecahedron and a biaugmented trigonal prism, while for

hydroxy-3-methoxybenzylidene)-6-(hydroxymethyl)picolinohydrazide (LH3),12 which allowed us to assemble tetranuclear compounds made up of two independent dimeric subunits (Scheme 1a).12 In these complexes the pendant −CH2OH groups of the ligand were still not deprotonated. It was of interest to examine the effect of full deprotonation of the ligand LH3 on the nuclearity of the ensemble. We were aware of the fact that the deprotonated [−CH2O]− group would be able to participate in a bridging coordination mode and hence would facilitate the formation of a larger polynuclear complex. Accordingly, in the reaction of LH3 with lanthanide metal salts in the presence of an excess of triethylamine (1:1.5:5), the hexanuclear complexes [{Ln 6 (L) 2 (LH) 2 }(μ 3 -OH) 4 ][MeOH]p[H2O]q[Cl]4·xH2O·yCH3OH (1, Ln = Gd(III), p = 4, q = 4, x = 8, y = 2; 2, Ln = Dy(III), p = 2, q = 6, x = 8, y = 4; 3, Ln = Tb(III), p = 2, q = 6, x = 10, y = 4; 4, Ln = Ho(III), p = 2, q = 6, x = 10, y = 2) (Scheme 1b) were formed. This reaction is sensitive to the reaction conditions. Thus, in the presence of controlled amounts of base and pivalic acid as the coligand tetranuclear derivatives were obtained.12 On the other hand, the use of only lanthanide nitrates as the precursors resulted in an insoluble precipitate in the presence of an excess of base. The reaction of lanthanide chlorides with LH3 in the presence of an excess of base and pivalic acid afforded tetranuclear compounds in very low yields. Other products could not be isolated. Only under the reaction conditions shown in Scheme 1b could we isolate the hexanuclear compounds. The molecular structures of all four complexes (1−4) were determined by single-crystal X-ray crystallography (vide infra). X-ray Crystallography. X-ray crystallographic analysis of 1−4 reveals that all of these complexes crystallize in the triclinic system in the centrosymmetric space group P1̅ with Z = 1. All of these complexes are tetracationic and contain four chlorides as the counteranions. Compounds 1−4 possess nearly similar structural arrangements with only minor structural variations (Scheme 1b and the Supporting Information). We describe herein the molecular structure of 2 as a representative example. The molecular structure of 2 is given in Figure 1; those of 1, 3, and 4 are given in the Supporting Information (Figures S1− S3). Selected bond parameters of 2 are summarized in Table 2. Other bond parameters, including those of 1, 3, and 4, are given in the Supporting Information (Tables S1−S3). The molecular structure of 2 reveals that the six Dy(III) ions are tightly held together by two triply deprotonated L3− ligands which are nonadentate (enolate form); each ligand holds 5024


fusion of three O-capped triangular Dy3 motifs which share vertices and edges triangular prism four coplanar Ln(III) ions in a trapezoid; two others lying above and below this plane

[Dy6(μ3-OH)3(μ3-CO3)(μ-OMe)(HL3)6(MeOH)4(H2O)2]·3MeOH·2H2O

5025

8- and 9-coordinated (see Figure 3 and Table S4)

8-coordinated (distorted dodecahedral coordination) and 9-coordinated (monocapped square antiprismatic)

8-coordinated

8-coordinated (hula-hoop-like) 8-coordinated (distorted square antiprismatic) and 9-coordinated (distorted monocapped square antiprismatic) 8-coordinated 8- and 9-coordinated 8-coordinated (distorted square antiprismatic)

8-coordinated (distorted-dodecahedral) 8- and 9-coordinated 8-coordinated (distorted square antiprismatic), 9-coordinated (distorted monocapped square antiprismatic) 8-coordinated

coordination no. (local geometry anound Ln(III) centers) magnetic properties

ref

19l

this work

SMM; Ueff = 46.2 K, τ0 = 2.85 × 10−7 s (Dy analogue shows only SMM behavior)

19k

19j

19g 19h 19i

19e 19f

19d

19a 19b 19c

magnetism not measd

non-SMM

SMM; Ueff = 5.6, 37.9 K, τ0 = 4.2 × 10−5, 3.8 × 10−6 s SMM; Ueff = 76 K, τ0 = 1.2 × 10−6 s SMM; Dy6 analogue reveals out-ofphase ac susceptibility signals without showing peak maxima SMM; Ueff = 200 K, τ0 = 1.5 × 10−9 s SMM; Ueff = 9.6 K, τ0 = 2 × 10−6 s out-of-phase ac susceptibility displays frequency dependence without resolving any peak maxima SMM; Ueff = 3.2 K, τ0 = 2.8 × 10−5 s

SMM; Ueff = 56 K, τ0 = 6.6 × 10−6 s SMM; Ueff = 29 K, τ0 = 4.2 × 10−6 s SMM; Ueff = 33.9, 40.7 K, τ0 = 5.8 × 10−8, 1.2 × 10−7 s

Abbreviations: H2L1, (E)-N′-(2-hyborxy-3-methoxybenzylidene) pyrazine-2-carbohydrazide; H2apovh, N′-(amino(pyridin-2-yl)methylene)-o-vanilloyl hydrazine; H3L2, formed by the in situ condensation of 2,6-diformyl-4-methylphenol and ethanolamine; H2L3, (E)-N′-(2-hydroxybenzylidene) pyrazine-2-carbohydrazide; H2pvph, 3-methoxysalicylaldehyde picoloyl hydrazone; H3tea, triethanolamine; Hchp, 6-chloro-2-hydroxypyridine; HL4, o-vanillin; H2L5, 2-hydroxymethyl-6-methoxyphenol; H2ava, 4-hydroxy-4-(2-hydroxy-3-methoxyphenyl)butan-2-one; H3L5, 1,3-bis(salicylideneamino)-2-propanol; L6, 4amino-3,5-dimethanyl-1,2,4-triazole; H2L7, N,N′-bis(3-methoxysalicylidene)(propylene-2-ol)-1,3-diamine; H2L8, N,N′-bis(salicylidene)(propylene-2-ol)-1,3-diamine.

a

[Ln6(L7)4(OH)4(MeOH)4][Cl]2·4MeOH (Ln = Nd, Tb), [Eu6(L8)4(OH)4(MeOH)2(EtOH)2(H2O)2][Cl]2·3EtOH·H2O, [Er6(L8)4(OH)4(EtOH)2(H2O)2][Cl]2·2EtOH·MeOH·H2O [{Ln6(L)2(LH)2}(μ3-OH)4][MeOH]p[H2O]q[Cl]4·xH2O·yCH3OH (1, Ln = Gd(III), p = 4, q = 4, x = 8, y = 2; 2, Ln = Dy(III), p = 2, q = 6, x = 8, y = 4; 3, Ln = Tb(III), p = 2, q = 6, x = 10, y = 4; 4, Ln = Ho(III), p = 2, q = 6, x = 10, y = 2) hexanuclear [Dy6(OH)4]14+ core consisting of triangular trimeric motifs [Dy3(OH)]8+ fused to each other at the edge hexanuclear [Dy6(OH)4]14+ core consisting of triangular trimeric motifs [Dy3(OH)]8+ fused to each other at the edge

edge-to-edge arrangement of two Dy3 (Dy3(μ3-O)(μ3-OH)) triangles octahedron

[Dy6(L5)4(μ3-OH)4(CH3OH)2(NO3)2]·6CH3CN

[Ln6(μ6-O)(μ3-OH)8(L6)4(H2O)14][Cl]8·2L6·6H2O (Ln = Er, Ho, Dy) and [Ln6(μ6-O)(μ3OH)8Cl2(L6)4(H2O)14][Cl]6·9H2O (Ln = Gd, Tb)

μ3-OH-capped triangular units μ3-OH capped triangular units wheel

[Dy6(μ3-OH)4L44L52-(H2O)9Cl][Cl]5·15H2O [Dy6(μ3-OH)4L44(avn)2 (NO3)4(H2O)4](NO3)2·H2O·3(CH3)2CO [Ln6(teaH)6(NO3)6]·8MeOH [Ln = Dy, Gd)

[Dy6(ovph)4(Hpvph)2Cl4 (H2O)2(CO3)2]·CH3OH·H2O·CH3CN [Ln6(teaH)2(teaH2)2(CO3) (NO3)2(chp)7(H2O)](NO3)·4.5MeOH·1.5H2O (Ln = Gd, Tb, Dy)

trigonal prism covalently linked pair of Dy3 clusters two Dy3 triangles fused in an edge-to-edge manner

core topology

[Dy6(OAc)3(μ3-CO3)2(L1)5(LH1)(MeOH)2]·4H2O·5MeOH·EtOH [Dy6(apovh)4(Hapovh)4 (CO3)2(SCN)2]·6CH3CN·8CH3OH·2H2O [Dy6L24(μ4-O)(NO3)4(CH3OH)]·CH3OH

compounda

Table 3. Structural and Magnetic Features of Hexanuclear Lanthanide Asemblies

Inorganic Chemistry Article


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Table 4. Comparison of the Average Bond Lengths (Å) and Bond Angles (deg) in the Hexanuclear Core Reported Here along with Those Belonging to Similar Cores Reported in the Literature [Ln6(L7)4(OH)4(MeOH)4][Cl]2·4MeOH [{Ln6(L)2(LH)2}(μ3-OH)4][MeOH]p[H2O]q; 1−4

Ln−-μ3-Ohydroxy

Ln−μ2-Ophenolate

Ln−Nimine

Ln−μ3-Ohydroxy−Ln

ref

2.442 2.426

2.341 2.369

2.542 2.529

95.4−110.1 96.9−109.4

19l this work

Figure 5. (a) Temperature dependence of χMT for 1−4 with detail of the χMT values found at the limits and the value anticipated at 300 K. (b) Field dependence of the magnetization obtained at 2 K (the solid lines are just guides for the eye). The dotted line corresponds to the calculated behavior for six noninteracting Gd(III) ions (see text).

It is interesting to compare the structural and magnetic features of the current family of Ln6 complexes with those known in the literature (Table 3).19 Generally, it is seen that most of the hexanuclear cores known in the literature involve O-capped triangular trimeric motifs which are fused to each other either at the vertices or at the edges. In addition, other topologies such as the wheel,19i the triangular prism,19a,e and the octahedron19k are also known. It is important to mention that, in spite of using a completely different ligand system, the core topology reported in this paper is similar to that reported by Jones et al.19l The latter did not perform magnetic measurements on their complexes. The metric parameters of the current Ln6 family are very similar to those of other Ln6 compounds that possess structurally similar cores (Table 4). Magnetic Properties. The temperature dependence of χMT for 1−4 (χM stands for the molar magnetic susceptibility) is given in Figure 5. The χMT values obtained at 300 K are in agreement with the values anticipated for the six noninteracting Ln ions (see Figure 5 for details). For the four derivatives the value of χMT falls between 300 and 2 K with a faster drop at low temperature. While for 2−4 this decrease can be attributed to the crystal field effect,20 the decrease observed for the Gd derivative 1 below 50 K reveals Gd−Gd exchange interactions. A fit of the Curie−Weiss law to the temperature dependence of 1/χGd (where χGd is the magnetic susceptibility per Gd) yielded Θ = −1.5 K and C = 7.90, thus confirming the occurrence of weak antiferromagnetic interactions. The field dependence of the magnetizations for 1−4 (Figure 5b) exhibit a fast increase for weak fields and a more gradual increase for stronger fields. For 1 the magnetization is compared to the behavior for six S = 7 /2 spins (with g = 2) in the absence of exchange interactions calculated by the Brillouin function21 (dotted line in Figure 5b). It can be seen that the experimental curve runs below the calculated curve for any field below 45 kOe, which is further evidence for weak antiferromagnetic Gd−Gd interactions. For compounds 2−4 the magnetic anisotropy of the corresponding Ln ions also contributes to the gradual increase of M with the field.

Figure 6. (a) Temperature dependence of χM′ and χM″ ac susceptibility signals for 2 (measured with Hac = 3 Oe and Hdc = 5 kOe). (b) Arrhenius plot and best fit (red line), yielding Δ/kB = 46.2 K and τ0 = 2.85 × 10−7 s.

the nonacoordinated Ln spherical capped square antiprism, spherical tricapped trigonal prism, and muffin geometries delimitate the experimental shape. 5026


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material is available free of charge via the Internet at http:// pubs.acs.org. X-ray crystallographic data in CIF format for 1−4 have also been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 977413−977416. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, +441223-336033; e-mail, [email protected]; web, http:// www.ccdc.cam.ac.uk).

The existence of SMM-type behavior for 2−4 was investigated by ac susceptibility measurements; only the Dy derivative exhibited evidence for slowly relaxing magnetization (Figure S5 (Supporting Information) for 3 and 4). For 2 the out-of-phase susceptibility (χM″) is nonzero below 15 K but no well-defined peak is obtained in the absence of a static magnetic field, because of quantum tunneling of the magnetization (QTM) (Figure S6 (Supporting Information)). Applied dc fields allowed reducing the QTM components and led to rounded maxima for χM″. The behavior observed in the presence of a dc field of 5 kOe (Figure 6a and Figure S7 (Supporting Information)) revealed a frequency dependence of the in-phase (χM′) and out-of-phase (χM″) ac signals. The blocking temperatures (defined as the peak of χM″ for a given frequency) is plotted in Figure 6b as ln τ versus 1/TB, where τ = 1/2πν is the corresponding relaxation time for a given frequency ν. The values obtained from a least-squares fitting of the Arrhenius law (τ = τ0 expΔ/kBT) are Δ/kB = 46.2 K and τ0 = 2.85 × 10−7 s, confirming SMM-type behavior. An evaluation of the distribution for the relation times from analysis of the Cole−Cole plot for 2 yielded a quite large α parameter (ca. 0.40; Figure S8 (Supporting Information)), which suggests a multipathway relaxation mechanism. This could be related to the structure of the compound that is made up by three different Dy(III) centers. All or at least two types of Dy could contribute to the observed overall slowly relaxing magnetization with slightly different characteristics, which hence lead to a distribution of relaxation times. Now it has been well established that the magnetization blocking barrier (Δ/k B ) is highly susceptible to the coordination environments, ligand strength, and their positioning in the coordination sphere as well as the intercenter interactions.22 The energy barrier for the magnetization reversal of Δ/kB = 46.2 K obtained here is moderately high and is in good agreement with the majority of hexanuclear Dy-based SMMs19a,b,e shown in Table 3. Table 3 also reveals that the higher energy barrier for magnetization reversal is associated mainly with the octacoordinated Dy(III) ions.

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*E-mail for J.-P.S.: [email protected]. *E-mail for V.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Department of Science and Technology, New Delhi, India, for financial support. S.D., S.H., A.D., and S.B. thank the CSIR of India for Senior Research Fellowships. V.C. is grateful to the Department of Science and Technology for a J. C. Bose National Fellowship. J.-P.S. acknowledges the CEFIPRA/IFCPAR (Indo-French Center for the Promotion of Advanced Research) for support.

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REFERENCES

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CONCLUSION Ligand design and its utilization in the preparation of moleculebased magnets is an important subject. Subtle changes in the reaction conditions, even when the same ligand is employed, can lead to widely different results. This message is brought home very well in the current effort, which showed that the ligand N′-(2-hydroxy-3-methoxybenzylidene)-6(hydroxymethyl)picolinohydrazide (LH3) can afford homonuclear [Ln4]12 or [Ln6] complexes under slightly different reaction conditions. The [Ln]6 family described here possess a [Ln6(OH)4]14+ core consisting of four fused [Ln3(OH)]8+ subunits. A detailed magnetic study of all complexes and particularly the gadolinium analogue revealed very weak antiferromagnetic interactions among the lanthanide ions. ac susceptibility measurements showed that only the dysprosium analogue is a SMM with the effective energy barrier Δ/kB = 46.2 K and relaxation time pre-exponential parameter τ0 = 2.85 × 10−7 s.

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AUTHOR INFORMATION

Corresponding Authors

ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving molecular structures of 1, 3, and 4, detailed structural parameters for compounds 1, 3, and 4, magnetic data for 2, and crystallographic data for 1−4. This 5027


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Molecular Magnets Based on Homometallic Hexanuclear Lanthanide

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